This is a trick question. Some species always have five rays (for example, Asterias), some always have more (sunstars typically have 9 or more), and some are variable within their species (the bat star Asterina miniata has between 4 and 9, with 5 being the most common).
But why five? Echinoderms evolved originally from bilateral ancestors (animals with distinct right and left sides, like us) and still have bilaterally symmetric larvae. Radial symmetry probably evolved back in the days when being a sessile (non-moving) suspension-feeder was in style. When you're stuck in one place, having a front and a back just means you're always facing the wrong way. Some early forms had three sides, but five eventually became the dominant form. When being a sessile suspension-feeder went out of style (probably because of the evolution of fish with jaws, which turned all these sessile creatures into a buffet), many of the sessile forms went extinct. Crinoids are the closest in form to those groups. Radial symmetry never went away after that, although sea cucumbers (another echinoderm group) have become more and more bilateral in form.
|Below: Pycnopodia helianthoides, the sunflower star. Sunflower stars start with around 9 arms and add more as they grow. Adults typically have over 20.
Above: Asterias vulgaris, a species that always has five rays. This just goes to show that biology makes exceptions to everything. (From Martinez's Marine Life of the North Atlantic.)
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Last updated May 10, 2000.